Using Algebraic Equation in Flash

[danhodkinson]

ok i'll describe this as simple as i can.
in AS3 i have a start value of x = 0 and an end value of x = 400 with a tween to get it from start to finish by using x += 400.
however if you double click the button which triggers the function on the 2nd click it takes that x position and adds 400 onto it so everything goes out of sync.
i am now trying to make the end value always equal to the appropriate place it should be ie if x = 250 it should only add on 150 and if x = 450 it should = 800.
can anybody offer me an equation to help me as i can't think of it.
thanks
Dan

[newblack]

400 - x gives you the amount to tween- most tweening packages offer relative (this scenario) and absolute coordinates; it might be possible to explicitly set 400 as your target.